Joe Berlinger sheds light on a notroious Boston crime boss, and the cracks in the American criminal justice system.

If access is a documentary filmmaker’s best friend, Joe Berlinger must have felt awfully lonely during the making of “Whitey,” which chronicles the arrest and trial of the infamous Boston crime boss James Joseph Bulger Jr. Unlike the unfettered access to defendants and courtrooms he enjoyed on his now-classic “Brother’s Keeper” and “Paradise Lost” trilogy, the director was here barred from Bulger’s trial (where cameras were prohibited in the courtroom) and Bulger himself (on lockdown in a Massachusetts penitentiary). But from that seemingly impossible position, “Whitey” emerges as yet another of Berlinger’s gripping, irony-laced snapshots of the American criminal justice system, in which his eponymous subject comes across as an incontestable monster who may, nevertheless, also be an unwitting patsy. Tightened by 20 minutes since its Sundance premiere, the docu opens day-and-date today via Magnolia.

Although Berlinger has been vocal about wanting to transition into dramatic features (and recently announced one such project), “Whitey” reaffirms that he has few peers when it comes to digging under the skin of sensational legal cases and the communities affected by them. Here, rather than dwelling on the case’s limitations, he makes the most of what he does have: plentiful interviews with attorneys for both sides, retired law enforcement officers and journalists long on the Bulger case; and candid testimonies from the spouses, siblings and children who lost loved ones at the hands of Bulger and other members of his notorious Winter Hill crime gang. In many instances, Berlinger records his subjects driving through the South Boston neighborhoods the gang once ruled with an iron fist — the streets safer now, yet still haunted by the past.

“Whitey” opens with a particularly chilling remembrance from former liquor-store owner Stephen Rakes, who recounts the night Bulger and right-hand henchman Kevin Weeks (also interviewed extensively here) showed up on his doorstep and introduced themselves as his new business partners. It’s a story made more unsettling by the fact that Rakes doesn’t live to tell it in open court, turning up dead from a supposedly unrelated case of poisoning midway through Berlinger’s shoot. With his inch-thick Bahston accent and tough, working-class disposition, Rakes is one of many interviewees who seems to have walked right out of the frames of “The Town” (or, perhaps, the forthcoming “Black Mass,” starring Johnny Depp as Bulger), but there’s nothing the least bit caricatured about the pain and anger these men and women have carried with them for the two decades since Bulger fled an impending FBI indictment and seemed to disappear into the ether. (He was eventually discovered living under an alias, in otherwise plain sight, in Santa Monica in 2011).

How did Bulger manage to stay several steps ahead of the feds for so much of his career? It is this question, above and beyond the rather foregone matter of his guilt, that becomes central to both the trial and Berlinger’s film. To hear the government tell it, Bulger and his associate Steve Flemmi were both longtime FBI informants, given long leashes by the bureau in exchange for tips that led to high-profile arrests and convictions (including of several members of the Angiulo crime family). But Bulger himself paints a different picture: a sordid portrait of FBI agents on the take and on the make, like Bulger and Flemmi’s supposed handler, John Connolly, who allegedly supplied violent criminals with privileged information in exchange for cold hard cash. And it is the great coup of “Whitey” that we get to hear Bulger (who never took the stand in court) tell this tale in his own inimitable words — plainspoken, even a touch folksy — via audio recordings of his jailhouse phone conversations with lawyer J.W. Carney Jr.

The counter-argument goes that Bulger, who always fancied himself a Robin Hood-style gentleman gangster, is tending to matters of legacy here and can’t bear the thought of being regarded by history as a rat — the ultimate gangland transgression. But nothing in “Whitey” is nearly so cut-and-dried, and Berlinger excels at pushing past the mainstream media groupthink (which largely accepted the FBI’s version of events at face value) to reveal a sea of corruption as murky as the Charles River. Experts attest that the contents of Bulger’s FBI file are inconsistent with the kind of high-level informant he supposedly was. Mysteriously redacted documents emerge from the depths of secret FBI vaults. And the line separating the law of the land from the law of the street becomes hopelessly indistinct. Time and again Bulger’s prosecutors (the FBI itself declined to be interviewed for the film) remind us that American justice system is not on trial here — but maybe, “Whitey” compellingly argues, it should be.

In lieu of actual courtroom footage, Berlinger re-creates key excerpts of the trial using an elegant mixture of artist sketches, onscreen text and voice performances. Composer Wendy Blackstone contributes greatly to the tense, unrelenting mood with a driving electric-guitar score.